This invention relates to sealing glass suitable for fusing together soda-lime-silica glass at low temperatures.
Recently, many types of panel display devices utilizing gas discharge, fluorescence and liquid crystal have been developed and manufactured. In such display devices, panel units are generally used wherein a plurality of glass plates are disposed to oppose each other with a predetermined spacing therebetween and the peripheries of the glass plates are properly sealed. From the standpoint of price, the glass plates of the panel unit are generally made of soda-lime-silica glass and their surfaces are covered with transparent electroconductive films or subjected to desired surface treatment. Many types of excellent organic binders have been used in most cases. However, where an organic binder is used for the panel of the liquid crystal display device, an active gas evolved in the package from the binder or moisture contained in air leaked into the package through the binder chemically reacts with the liquid crystal, thus impairing its characteristics. For this reason, where a high degree of reliability is desired, it is necessary to fuse together the glass plates with inorganic low melting point glass free from moisture absorbing characteristics.
In order to fuse and seal the peripheries of the glass plates of the panel unit without deteriorating the electrode films, surface treated films or the state of surface treated glass plates, it is advantageous that the sintering temperature utilizing in the fusion operation should be less than 400.degree. C. and that the thermal expansion coefficient (.alpha.) of the sealng glass should be comparable with that of the glass plates.
Low melting point sealing glass for this purpose has been proposed, see, for example, Japanese patent publication No. 44-22189, Japanese laid open patent specification Nos. 50-44207 and 50-84614. The low melting glass disclosed in Japanese patent publication No. 44-22189 has a softening point lower than 400.degree. C., but its thermal expansion coefficient differs greatly from .alpha.=92.times.10.sup.-7 /C..degree. of the soda-lime-silica glass so that it is impossible to manufacture high quality panel units. The sealing glasses disclosed in the other two references have high softening points so that it is impossible to use them at or below 400.degree. C.
When one tries to lower the softening point of low melting glass, its thermal expansion coefficient generally tends to increase. However, when the difference in the thermal expansion coefficient between the soda-lime-silica glass (.alpha.=92.times.10.sup.-7 /C..degree.) plates to be fused together and the sealing glass increases beyond a certain limit, the sealing glass would crack while the temperature is decreased from the fusion temperature to room temperature, owing to the difference in the contractions between the sealing glass and the glass plate. Accordingly, it is highly desirable to develop sealing glass that can decrease the difference in the thermal expansion coefficients and having a softening point less than 400.degree. C.